Line-of-sight multi-input multi-output enabled multibeam satellite system

ABSTRACT

The present teachings disclose a multibeam satellite system and methods that can achieve orthogonality between spatially multiplexed signals in a multi-input multi-output (MIMO) configuration when operating in line-of-sight (LOS) uplink and downlink channels on the feeder link side, using essentially a common spot beam. The teachings maximize a MIMO capacity across multiple frequency bands by disclosing an antenna array geometry for disposition on-board a single satellite and for a ground segment.

FIELD

Spatial orthogonality of independent signals transmitted in MIMO-enabledsatellite systems with LOS channels is achieved, when these signals usethe same time-frequency resources. A single satellite with multiplereflector antennae may be used to minimally increase a cost of thesatellite system. For a given geographic area, more gateways may beplaced with acceptable interference levels among them. In someembodiments, a reliance on diversity gateways is reduced.

BACKGROUND

Prior Art MIMO systems require channels that are scatter-rich, such astypically found in terrestrial wireless networks. Some prior artsatellites systems use MIMO for low-earth orbit (LEO) applications asthey would still benefit from scatter-rich environment, resulting fromRicean fading with low Rice factor. Some other prior art satellitessystems use multiple satellites with one MIMO antenna disposed on eachsatellite, a very costly alternative. In contrast, LOS conditions,rather than scatter-rich, are encountered in geostationary satellitesystems traditionally limiting the usefulness of MIMO.

BRIEF SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that is further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

The present teachings disclose a multibeam satellite system that canachieve orthogonality between spatially multiplexed signals in amulti-input multi-output (MIMO) configuration when operating inline-of-sight (LOS) uplink and downlink channels on a feeder link side,using essentially a common spot beam. The teachings maximize a MIMOcapacity across multiple frequency bands by disclosing an antenna arraygeometry for disposition on-board a single satellite and for a groundsegment.

A system of one or more computers can be configured to performparticular operations or actions by virtue of having software, firmware,hardware, or a combination of them installed on the system that inoperation causes or cause the system to perform a method for providingMulti-Input Multi-Output (MIMO) feeder uplinks for a satellite. Themethod including separating X satellite antennae on the satellite,pre-interference processing Y gateway uplink Tx signals to generate Yantennae uplink signals that minimize channel interference for a MIMOreception at the X satellite antennae, and disposing Y ground antennaesuch that a MIMO transmission of the Y antennae uplink signals by the Yground antennae is spatially orthogonal upon the MIMO reception at the Xsatellite antennae, where X is greater than 1, Y is greater than 1, theX satellite antennae are dominant line-of-sight (LOS) of the Y groundantennae, and a channel capacity of the MIMO transmission is greaterthan a channel capacity of Y Single-Input Single-Output (SISO)transmissions.

In some embodiments, the method for the uplinks may include one or moreof the following. The method may include transmitting the Y antennaeuplink signals as the MIMO transmission; and receiving the MIMOtransmission at the satellite. The method where the pre-interferenceprocessing is based on one or more of, a weighted or non-weightedversion of, a zero-forcing (ZF) criteria, a minimum mean-square error(MMSE) criteria, or a regularized ZF (RZF) criteria. The method wherethe Y ground antennae are spaced within an acceptable range of anoptimal position. The method where the satellite includes a GEOsatellite having a bent-pipe design, an on-board processing design, atransparent payload design, a regenerative payload design or acombination thereof. The method where X and Y are equal. The methodwhere the pre-interference processing mitigates interference in apresence of an additive white Gaussian noise (AWGN) vector. The methodwhere the pre-interference processing is based on a linear operationmultiplying the Y antennae uplink signals with a matrix or a linearcombination of the Y antennae uplink signals. The method where theground antennae are interconnected via a fiber or microwave link. Themethod where the pre-interference processing is based on high-qualitychannel state information (CSI) about the propagation of the MIMOtransmission. Other technical features may be readily apparent to oneskilled in the art from the following figures, descriptions, and claims.

A system of one or more computers can be configured to performparticular operations or actions by virtue of having software, firmware,hardware, or a combination of them installed on the system that inoperation causes or cause the system to perform a method for providingMulti-Input Multi-Output (MIMO) feeder downlinks for a satellite. Themethod including separating X satellite antennae on the satellite,disposing Y ground antennae such that a MIMO transmission by the Xsatellite antennae is spatially orthogonal upon a MIMO reception of Yantennae downlink signals by the Y ground antennae, andpost-interference processing the Y antennae downlink signals to generateY gateway downlink signals that minimize channel interference for theMIMO reception at the Y ground antennae, where X is greater than 1, Y isgreater than 1, the Y ground antennae are dominant line-of-sight (LOS)of the X satellite antennae, and a channel capacity of the MIMOtransmission is greater than a channel capacity of X Single-InputSingle-Output (SISO) transmissions.

In some embodiments, the method for the downlinks may include one ormore of the following. The method may include transmitting the MIMOtransmission via the X satellite antennae; and receiving the MIMOtransmission as the Y antennae downlink signals. The method where the Yground antennae are spaced within an acceptable range of an optimalposition. The method where the satellite includes a GEO satellite havinga bent-pipe design, an on-board processing design, a transparent payloaddesign, a regenerative payload design or a combination thereof. Themethod where X and Y are equal. The method where the post-interferenceprocessing mitigates interference in a presence of an additive whiteGaussian noise (AWGN) vector. The method where the post-interferenceprocessing is based on a linear operation multiplying the Y antennaedownlink signals with a matrix or a linear combination of the Y antennaeuplink signals. The method where the post-interference processing isbased on one or more of, a weighted or non-weighted version of, azero-forcing (ZF) criteria, a minimum mean-square error (MMSE) criteria,or a regularized ZF (RZF) criteria. The method where the ground antennaeare interconnected via a fiber or microwave link. The method where thepost-interference processing is based on high-quality channel stateinformation (C SI) about the propagation of the MIMO transmission. Othertechnical features may be readily apparent to one skilled in the artfrom the following figures, descriptions, and claims.

Additional features will be set forth in the description that follows,and in part will be apparent from the description, or may be learned bypractice of what is described.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features may be obtained, a more particular descriptionis provided below and will be rendered by reference to specificembodiments thereof which are illustrated in the appended drawings.Understanding that these drawings depict only typical embodiments andare not, therefore, to be limiting of its scope, implementations will bedescribed and explained with additional specificity and detail with theaccompanying drawings.

FIG. 1A illustrates a MIMO-enabled feeder link for multibeam satellitesystems in LOS for a 2×2 case according to some embodiments.

FIG. 1B illustrates a MIMO-enabled feeder link for multibeam satellitesystems in LOS for a 3×3 case according to some embodiments.

FIG. 2 illustrates an aspect of the subject matter in accordance withone embodiment.

FIG. 3 illustrates a maximum MIMO capacity that is achieved fordifferent choices of the number of transmit and receive antennaeaccording to some embodiments.

FIG. 4 illustrates a capacity of a 2×2 MIMO as a function of distancebetween the ground antennae on an uplink in the forward directionaccording to some embodiments.

FIG. 5 illustrates a capacity of a 3×3 MIMO as a function of distancebetween the ground antennae on an uplink in the forward directionaccording to some embodiments.

FIG. 6 plots a three-dimensional normalized capacity as a MIMO pair ofgateways experience a rainfall event on an uplink in the forwarddirection according to some embodiments.

FIG. 7 illustrates a block diagram of ground-based linearpre-interference processing for spatial distinguishability on an uplinkportion of a feeder link in a forward direction according to variousembodiments.

FIG. 8 illustrates a capacity of a 2×2 MIMO as a function of distancebetween the ground antennae on a downlink in the return directionaccording to some embodiments.

FIG. 9 illustrates a capacity of a 3×3 MIMO as a function of distancebetween the ground antennae on a downlink in the return directionaccording to some embodiments.

FIG. 10 plots a three-dimensional normalized capacity as a MIMO pair ofgateways experience a rainfall event on a downlink in a return directionaccording to some embodiments.

FIG. 11 illustrates a block diagram of ground-based linearpost-interference processing for spatial distinguishability on adownlink portion of a feeder link in a return direction according tovarious embodiments.

FIG. 12 illustrates a Method 1200 for providing Multi-Input Multi-Output(MIMO) feeder uplinks for a satellite in accordance with one embodiment.

FIG. 13 illustrates a method 1300 for providing Multi-Input Multi-Output(MIMO) feeder downlinks for a satellite in accordance with oneembodiment.

Throughout the drawings and the detailed description, unless otherwisedescribed, the same drawing reference numerals will be understood torefer to the same elements, features, and structures. The relative sizeand depiction of these elements may be exaggerated for clarity,illustration, and convenience.

DETAILED DESCRIPTION

The present teachings may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as SMALLTALK, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present invention, as well as other variations thereof, means that afeature, structure, characteristic, and so forth described in connectionwith the embodiment is included in at least one embodiment of thepresent invention. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

Introduction

The present teachings disclose a multibeam satellite system that canachieve orthogonality between spatially multiplexed signals in amulti-input multi-output (MIMO) configuration when operating inline-of-sight (LOS) channels, using essentially a common spot beam. Theteachings maximize a MIMO capacity across multiple frequency bands bydisclosing an antenna array geometry for disposition on-board a singlesatellite and for a ground segment.

In some embodiments, linear pre-processing at the gateways spatiallyseparates the multiplexed signals without requiring matrix processingonboard the satellite for the uplink of the forward direction. In someembodiments, for the downlink, linear post-processing at the gatewaysmay spatially separate the multiplexed signals without requiring matrixprocessing onboard the satellite. The gateway-based linear processingenables MIMO in LOS with existing satellite bent-pipe architectures.

Significant gains are demonstrated when compared with systems that donot use this MIMO method, for example, about 125% improvement incapacity for a 2×2 MIMO over a conventional single-input single-output(SISO) system. The present teachings provide soft diversity to gainimmunity against heavy rain attenuation.

Although the examples herein are based on a Geosynchronous Earth Orbit(GEO) satellite, they are also applicable to Low Earth Orbit (LEO) andMedium Earth Orbit (MEO) satellites as long as LOS channels, inparticular, dominant LOS channels are used. In a dominant LOS channel, afree space signal from the transmitter to the receiver is stronger thana scattered space signal from the transmitter to the receiver.

FIG. 1A illustrates a MIMO-enabled feeder link for multibeam satellitesystems in LOS for a 2×2 case in one embodiment.

FIG. 1A illustrates an exemplary satellite network 100 that implementsfeeder links using a 2×2 MIMO. Satellite network 100 includes twosatellite antennae 104 at a satellite (not shown) separated by adistance d_(S) that are radiating two highly overlapping beams 112. Aground portion of the satellite network 100 consists of two gatewayantennae 102, separated by a distance d_(E) and inter-connected to adata processor 110 at a data processing center. Each gateway antenna 102serves both satellite antennae 104 simultaneously. Both uplinks 106 anddownlinks 108 on the feeder side are provided by the beams 112. In someembodiments, the gateway antennae 102 are multi-feed antennae. In someembodiments, the satellite antennae 104 are multi-feed antennae.

FIG. 1B illustrates a MIMO-enabled feeder link for multibeam satellitesystems in LOS for a 3×3 case in one embodiment.

FIG. 1B illustrates an exemplary satellite network 100 that implementsfeeder links using a 3×3 MIMO. Satellite network 100 includes threesatellite antennae 104 at a satellite (not shown) separated by adistance d_(S) that are radiating three highly overlapping beams 112. Aground portion of the satellite network 100 consists of three gatewayantennae 102, separated by a distance d_(E) and inter-connected to adata processor 110 at a data processing center. Each gateway antenna 102serves the three satellite antennae 104 simultaneously. Both uplinks 106and downlinks 108 on the feeder side are provided by the beams 112. Thegateway antennae 102 are arranged on a straight line for illustrationonly; other arrangements are possible such as forming a triangle.

FIG. 2 illustrates mathematical equations for MIMO feeder linksaccording to various embodiments.

A MIMO uplink operation—gateway antennae 102 to satellite antennae104—may be mapped by an expression 202, an expression 204, an expression206, an expression 208, an expression 210, an expression 212, anexpression 214, and an expression 216. A MIMO downlinkoperation—satellite antennae 104 to gateway antennae 102—may be mappedby an expression 218, an expression 220, an expression 222, anexpression 224, an expression 226, an expression 228, an expression 230and an expression 232.

FIG. 3 illustrates a maximum MIMO capacity that is achieved fordifferent choices of the number of transmit and receive antennaeaccording to some embodiments.

FIG. 3 illustrates a maximum capacity achievable for different counts oftransmit and receive antennae. The capacity plot 300 comprisescapacities for a SISO 302, a MIMO 2×2 304, a MIMO 3×3 306, and a MIMO4×4 308 configuration. In FIG. 3, Carrier to Noise Ratio (CNR) is scaledby the number of antennae to show that capacity increase is achievedpurely from gain in spatial degrees of freedom. The capacity plot 300illustrates channel capacity increases over a Single-Input Single-Output(SISO) system. The capacity plot 300 also illustrates that channelcapacity increases, more or less linearly, with an increase in the MIMOorder. For example, at 4 bits/sec/Hz, a 12-dB improvement is achieved byusing a 4×4 MIMO when compared to the SISO scenario.

Ground-Based Pre-Interference Processing for the Uplink

FIG. 4 illustrates a capacity of a 2×2 MIMO as a function of distancebetween the ground antennae on an uplink in the forward directionaccording to some embodiments.

Traditionally, LOS channels, encountered in geostationary earth orbit(GEO) satellite systems, have been disregarded as being able to achievethe maximum MIMO capacity. Therefore, the applications of MIMO havemostly focused on scatter-rich channel environments which are typicallyfound in terrestrial wireless networks. However, it is possible tomaximize the spatial multiplexing gain of MIMO-enabled feeder links ofmultibeam satellite systems in LOS. This is done through geometricoptimization of satellite/gateway antenna-array placements.

FIG. 4 illustrates a 2×2 uplink channel capacity plot 400 associatedwith a 2×2 MIMO configuration in LOS for a geostationary satellite onthe equator at orbital location of 97° W, with onboard antenna spacingof 6 m and a carrier frequency (f_(c)) in the E-band (81-86 GHz) on theuplink in the forward direction. The 2×2 uplink channel capacity plot400 includes capacities for a SISO uplink 402, a two SISO uplinks 404,and a 2×2 MIMO Uplink 406. For example, the gateway-pair center isplaced in North Las Vegas, Nev., with location of 36.4° N and 114.9° W.The CNR is 24 dB. Clearly, the placement of the gateway pair impacts theachievable capacity, cycling through phase alignment, with value atabout 10 bits/sec/Hz, to phase orthogonality, peaking at about 18bits/sec/Hz. The latter is the maximum MIMO capacity obtained when thetwo signal streams are spatially orthogonal, ensured by the optimallypositioned gateway pair. At gateway separation of about 36 Km, theproposed MIMO-enabled feeder link outperforms a state-of-the-art SISOfeeder link by a 125% and is 12.5% better than using two SISO feederlinks. Also evident is that the displacement of the gateway antennaewithin an acceptable range 408 of several kilometers around the optimalposition 410 of the gateway antennae causes very little reduction incapacity. The 2×2 MIMO Uplink 406 capacity is illustrated as a ribbon tocover the E-band frequency range (81-86 GHz).

FIG. 5 illustrates a capacity of a 3×3 MIMO as a function of distancebetween the ground antennae on an uplink in the forward directionaccording to some embodiments.

FIG. 5 illustrates a capacity plot associated with a 3×3 MIMOconfiguration in LOS for a geostationary satellite on the equator atorbital location of 97° W, with onboard antenna spacing of 6 m and acarrier frequency (f_(c)) in the E-band (81-86 GHz) on the uplink in theforward direction. The 3×3 uplink channel capacity plot 500 includes aSISO uplink 502, a three SISO uplinks 504, a 3×3 MIMO uplink 506, anacceptable range 508, and an optimal position 510. For example, thegateway-trio center is placed in North Las Vegas, Nev., with location of36.4° N and 114.9° W. The CNR is 24 dB. Clearly, the placement of thegateway trio impacts the achievable capacity, cycling through phasealignment, with value at about 11.2 bits/sec/Hz, to phase orthogonality,peaking at about 29 bits/sec/Hz. The latter is the maximum MIMO capacityobtained when the three signal streams are spatially orthogonal, ensuredby the optimally positioned gateway trio. At gateway separation of about17 Km, the proposed MIMO-enabled feeder link outperforms astate-of-the-art SISO feeder link by a significant 259% and is 20%better than using three SISO feeder links. Also evident is that thedisplacement of the gateways within acceptable ranges 508 around theoptimal positions 510 by several kilometers causes very little reductionin capacity.

Rainfall Effects on MIMO Feeder Uplinks

FIG. 6 plots a three-dimensional normalized capacity as a MIMO pair ofgateways experience a rainfall event on an uplink in the forwarddirection according to some embodiments.

One advantage of MIMO-enabled feeder links is inherent robustness torainfall events. This is due to the large gateway separations on theground, giving rise to diversity effects as the probability of a jointrainfall event affecting all the MIMO gateways is expected to be low.FIG. 6 displays the three-dimensional normalized capacity as the MIMOpair of gateways experience a rainfall events inducing attenuation overa wide range of values with CNR at 24 dB. The gateway placement isoptimized under clear-sky conditions and ground-based pre-interferenceprocessing is implemented to mitigate MIMO interference for the uplinkin the forward direction resulting from the differential attenuationsexperienced by both sites. The capacity reduction is minimized even asthe rain attenuation is 7.5 dB for one of the sites.

FIG. 7 illustrates a block diagram of ground-based linearpre-interference processing for spatial distinguishability on an uplinkportion of a feeder link in a forward direction according to variousembodiments.

The uplink pre-interference processing 700 includes a pre-interferenceprocessor 702 and a ground antennae 704. The pre-interference processor702 receives an antennae uplink signal 706 to generate a GW uplinksignal 708 for the ground antennae 704. An AWGN vector 710 may beapplied to the 708 and received as a MIMO transmission 712 aftertraversing the space between the ground antennae 704 and satelliteantennae (not shown).

In general, the system model for the feeder link on the uplink in theforward direction incorporates a MIMO system with N_(T) transmit gatewayantennae and N_(R) receive satellite antennae, accompanied by additivewhite Gaussian noise (AWGN) vector w_(u). This can be mathematicallyexpressed as Expression 202, where y_(S) is an N_(R)×1 complex-valuedvector received at the satellite antennae and x_(g) of size N_(T)×1, isthe vector of signals transmitted by the gateway antennae. In expression202, the matrix H_(u) represents the MIMO system in LOS whose entriesare computed based on the free-space wave propagation as expression 204where λ_(c)=c₀/f_(c) is the wavelength associated with the uplinkcarrier frequency f_(c), a_(m,n) is the propagation attenuation, ora_(m,n)=λ_(c)/(4πr_(m,n)and r_(m,n) is the distance between the nthtransmit antenna and the mth receive antenna.

The capacity C_(u) of such a MIMO-enabled system may be given byexpression 206, where I_(N) _(R) is the identity matrix and ρ_(u) is thecarrier-to-noise ratio (CNR) on the uplink in the forward direction.

In order to extract the spatial multiplexing gain with minimal increaseof the satellite complexity, the present teachings pre-correct thetransmitted symbol vector x_(g) at a pre-interference processor 702 viaa linear operation, for example, by multiplying the transmitted symbolvector x_(g) with a matrix G_(pre). Mathematically speaking, antennaeuplink signal 706

rather than x_(g), as in expression 208. y substituting expression 208into expression 202, the received vector of symbols experience acascaded channel effect marked by (H_(u)▪G_(pre)) per expression 210.

When the gateways are placed at their orthogonal positions G_(pre)=H_(u)^(H). This is so as the cascaded channel effect in this case,(H_(u)H_(u) ^(H)), is diagonal following the capacity-maximizinggeometric optimization of the satellite/gateway placement. This mayensure that the transmitted signals may be spatially distinguished atthe receive satellite antenna, without needing matrix multiplicationonboard the satellite.

If residual MIMO interference remains at the satellite, for example, dueto the gateways not being placed at orthogonal positions, other criteriamay be used to obtain G_(pre) such as those based on the zero-forcing(ZF), the minimum mean-square error (MMSE) criteria, or regularized ZF(RZF), expressed, respectively, expression 212, expression 214 andexpression 216. Weighting can also be applied to the expression 212,expression 214 and expression 216 to provide weighted sum capacity.

The ground-based linear pre-interference processing minimizes satellitecomplexity. This may enable MIMO in LOS with existing satellitebent-pipe design or one with a transparent payload. The MIMO-enabledfeeder links may operate with more advanced satellites with on-boardprocessing and ones with regenerative payloads. The pre-interferenceprocessing generates the transmit signals for the uplink. Thepre-interference processing may be inter-connected to the gateways viafiber or through microwave links, while ensuring time and phasealignment of the ground antennae. High-quality channel state information(CSI) about the propagation rays may be available at the gateways. Thiscan be done, for example, by deploying channel sounding usingcalibration sequences that are orthogonal.

Ground-Based Post-Interference Processing for the Downlink

FIG. 8 illustrates a capacity of a 2×2 MIMO as a function of distancebetween the ground antennae on a downlink in the return directionaccording to some embodiments.

FIG. 8 illustrates a capacity associated with a 2×2 MIMO configurationin LOS for a geostationary satellite on the equator at orbital locationof 97° W, with onboard antenna spacing of 6 m and f_(c) in the E-band(71-76 GHz) on the downlink in the return direction. The 2×2 downlinkchannel capacity plot 800 includes a SISO downlink 802, a two SISOdownlinks 804, a 2×2 MIMO downlink 806, an acceptable range 808, and anoptimal position 810. For example, the gateway-pair center is placed inNorth Las Vegas, Nev., with location of 36.4° N and 114.9° W. The CNR is24 dB. Clearly, the placement of the gateway pair impacts the achievablecapacity, cycling through phase alignment, with value at about 10bits/sec/Hz, to phase orthogonality, peaking at about 18 bits/sec/Hz.The latter is the maximum MIMO capacity obtained when the two signalstreams are spatially orthogonal, ensured by the optimally positionedgateway pair. At gateway separation of about 41 Km, the proposedMIMO-enabled feeder link outperforms a state-of-the-art SISO feeder linkby a tremendous 125% and is 12.5% better than using two SISO feederlinks. The displacement of the gateways within an acceptable ranges 808around the optimal positions 810 by several kilometers causes verylittle reduction in capacity. 2×2 MIMO downlink 806 is illustrated as aribbon to cover the E-band frequency range (71-76 GHz).

FIG. 9 illustrates a capacity of a 3×3 MIMO as a function of distancebetween the ground antennae on a downlink in the return directionaccording to some embodiments.

FIG. 9 displays the capacity associated with a 3×3 MIMO configuration inLOS for a geostationary satellite on the equator at orbital location of97° W, with onboard antenna spacing of 6 m and f_(c) in the E-band(71-76 GHz) on the downlink in the return direction. The 3×3 downlinkchannel capacity plot 900 includes a SISO downlink 902, a three SISOdownlinks 904, a 3×3 MIMO downlink 906, an acceptable range 908, and anitem 910. For example, the gateway-trio center is placed in North LasVegas, Nev., with location of 36.4° N and 114.9° W. The CNR is 24 dB.Clearly, the placement of the gateway trio impacts the achievablecapacity, cycling through phase alignment, with value at about 11.2bits/sec/Hz, to phase orthogonality, peaking at about 29 bits/sec/Hz.The latter is the maximum MIMO capacity obtained when the three signalstreams are spatially orthogonal, ensured by the optimally positionedgateway trio. At gateway separation of about 36 Km, the proposedMIMO-enabled feeder link outperforms a state-of-the-art SISO feeder linkby a significant 259% and is 20% better than using three SISO feederlinks. Also evident is that the displacement of the gateways around theoptimal positions by several kilometers causes very little reduction incapacity.

Rainfall Effects on MIMO Feeder Downlinks

FIG. 10 plots a three-dimensional normalized capacity as a MIMO pair ofgateways experience a rainfall event on a downlink in a return directionaccording to some embodiments.

One advantage of MIMO-enabled feeder links is their inherent robustnessto rainfall events. This is due to the large gateway separations on theground, giving rise to diversity effects as the probability of a jointrainfall event affecting all the MIMO gateways is expected to be low.FIG. 10 displays the three-dimensional normalized capacity as the MIMOpair of gateways experience a rainfall events inducing attenuation overa wide range of values with CNR at 24 dB. The gateway placement isoptimized under clear-sky conditions and ground-based post-interferenceprocessing is implemented to mitigate MIMO interference for the downlinkin the return direction resulting from the differential attenuationsexperienced by both sites. The capacity reduction is minimized even asthe rain attenuation is 7.5 dB for one of the sites.

FIG. 11 illustrates a block diagram of ground-based linearpost-interference processing for spatial distinguishability on adownlink portion of a feeder link in a return direction according tovarious embodiments.

The downlink post-interference processing 1100 includes apost-interference processor 1102, satellite antennae 1104, antennaedownlink signals 1106 that are processed by the post-interferenceprocessor 1102 to generate GW downlink signals 1108. The satelliteantennae 1104 may receive and output MIMO receptions 1112. An AWGNvector 1110 may be applied to outputs of satellite antennae 1104 inorder to obtain antennae downlink signals 1106.

In general, the system model for the feeder link on the downlink in thereturn direction incorporates a MIMO system with N_(T) transmitsatellite antennae and N_(R) receive gateways antennae, accompanied byadditive white Gaussian noise (AWGN) vector w_(d). This can bemathematically expressed as expression 218, where y_(g) is an N_(R)×1complex-valued vector received at the gateway antennae and x_(s), ofsize N_(T)×1, is the vector of signals transmitted by the satelliteantennae. In expression 218, the matrix H_(d) represents the MIMO systemin LOS whose entries are computed based on the free-space wavepropagation as expression 220, where λ_(c)=c₀/f_(c) is the wavelengthassociated with the downlink carrier frequency f_(c), a_(m,n) is thepropagation attenuation, or or a_(m,n)=λ_(c)/(4πr_(m,n)) and r_(m,n) isthe distance between the nth transmit antenna and the mth receiveantenna.

The capacity C_(d) of such a MIMO-enabled system is given by expression222 where I_(N) _(R) is the identity matrix and ρ_(d) is the CNR on thedownlink in the return direction.

In order to extract the spatial multiplexing gain with minimal increaseof the satellite complexity, the received symbol vector y_(g) ispost-processed by the post-interference processor 1102 via a linearoperation involving multiplying by matrix G_(post). This ground-basedlinear post-interference processing is displayed in FIG. 11, focusing onthe case of equal number of transmit and receive antennae.Mathematically speaking, the present teachings generate GW downlinksignals 1108

instead, as in expression 224. By applying expression 224 to expression218, the received vector of symbols experience a cascaded channel effectthat is marked by (G_(post)▪H_(d)) , or expression 226.

When the gateways are placed at their optimal orthogonal positions, thenG_(post)=H_(d) ^(H). This is so as the cascaded channel effect in thiscase, (H_(d) ^(H)H_(d)=H_(d)H_(d) ^(H)), is diagonal following thecapacity-maximizing geometric optimization of the satellite/gatewayplacement. This choice of G_(post) ensures that the transmitted signalsmay be spatially distinguished at the receive gateway antenna, withoutneeding matrix multiplication onboard the satellite.

If residual MIMO interference remains due to reasons such as thegateways are not placed at their optimal orthogonal positions, othercriteria can be used to obtain G_(post) such as those based on thezero-forcing (ZF), the minimum mean-square error (MMSE) criteria, orregularized ZF (RZF), expressed, respectively, as expression 228,expression 230 and expression 232. Weighting may be applied toexpression 228, expression 230 and expression 232 to provide weightedsum capacity.

The proposed ground-based linear post-interference processing minimizesthe impact on the satellite complexity, enabling MIMO in LOS withexisting satellite bent-pipe design or one with a transparent payload.The proposed MIMO-enabled feeder links can operate with more advancedsatellite with on-board processing and ones with regenerative payloads.The post-interference processing is responsible for the generation ofthe received signals. Post-interference processing may beinter-connected to the gateways participating in the MIMO setupterrestrially via fiber or through microwave links, while ensuring timeand phase alignment of the ground antennae. High-quality channel stateinformation (CSI) about the propagation rays may be available at thegateways, for example, by deploying channel sounding using calibrationsequences that are orthogonal.

FIG. 12 illustrates a method for providing Multi-Input Multi-Output(MIMO) feeder uplinks for a satellite according to various embodiments.

FIG. 12 illustrates a method 1200 for providing MIMO uplinks for asatellite. In operation 1202, Method 1200 separates X satellite antennaeon the satellite. In operation 1204, Method 1200 pre-interferenceprocesses Y gateway uplink Tx signals to generate Y antennae uplinksignals that minimize channel interference for a MIMO reception at the Xsatellite antennae. In operation 1206, Method 1200 disposes Y groundantennae such that a MIMO transmission of the Y antennae uplink signalsby the Y ground antennae is spatially orthogonal upon the MIMO receptionat the X satellite antennae. In Method 1200, at 1208, X is greater than1, Y is greater than 1, the X satellite antennae are line-of-sight (LOS)of the Y ground antennae, and a channel capacity of the MIMO receptionat the X satellite antennae is greater than a channel capacity of XSingle-Input Single-Output (SISO) receptions at the X satelliteantennae. In operation 1210, Method 1200 transmits the Y antennae uplinksignals as the MIMO transmission. In operation 1212, Method 1200 mayreceive a MIMO transmission at the satellite.

FIG. 13 illustrates a method for providing Multi-Input Multi-Output(MIMO) feeder uplinks for a satellite according to various embodiments.

FIG. 13 illustrates a method 1300 for providing MIMO downlinks for asatellite. In operation 1302, method 1300 separates X satellite antennaeon the satellite. In operation 1304, method 1300 disposes Y groundantennae such that a MIMO transmission by the X satellite antennae isspatially orthogonal upon a MIMO reception of Y antennae downlinksignals by the Y ground antennae. In Operation 1306, method 1300transmits the MIMO transmission via the X satellite antennae. Inoperation 1308, method 1300 post-interference processes the Y antennaedownlink signals to generate Y gateway downlink signals that minimizechannel interference for the MIMO reception at the Y ground antennae. Inmethod 1300, at 1310, X is greater than 1, Y is greater than 1, the Xsatellite antennae are line-of-sight (LOS) of the Y ground antennae, anda channel capacity of the MIMO reception at the Y ground antennae isgreater than a channel capacity of Y Single-Input Single-Output (SISO)receptions at the Y ground antennae. In operation 1312, method 1300 mayreceive the MIMO transmission as the Y antennae downlink signals.

Having described preferred embodiments of a system and method (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artconsidering the above teachings. It is therefore to be understood thatchanges may be made in the embodiments disclosed which are within thescope of the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

What is claimed is:
 1. A method for providing Multi-Input Multi-Output(MIMO) feeder uplinks for satellite, the method comprising: separating Xsatellite antennae on the satellite; pre-interference processing Ygateway uplink Tx signals to generate Y antennae uplink signals thatminimize channel interference for a MIMO reception at the X satelliteantennae; and disposing Y ground antennae such that a MIMO transmissionof the Y antennae uplink signals by the Y ground antennae is spatiallyorthogonal upon the MIMO reception at the X satellite antennae, whereinX is greater than 1, Y is greater than 1, the X satellite antennae aredominant line-of-sight (LOS) of the Y ground antennae, and a channelcapacity of the MIMO transmission is greater than a channel capacity ofY Single-Input Single-Output (SISO) transmissions.
 2. The method ofclaim 1, further comprising transmitting the Y antennae uplink signalsas the MIMO transmission; and receiving the MIMO transmission at thesatellite.
 3. The method of claim 1, wherein the pre-interferenceprocessing is based on one or more of, a weighted or non-weightedversion of, a zero-forcing (ZF) criteria, a minimum mean-square error(MMSE) criteria, or a regularized ZF (RZF) criteria.
 4. The method ofclaim 1, wherein the Y ground antennae are spaced within an acceptablerange of an optimal position.
 5. The method of claim 1, wherein thesatellite comprises one Geosynchronous Earth Orbit (GEO) satellitehaving a bent-pipe design, an on-board processing design, a transparentpayload design, a regenerative payload design or a combination thereof.6. The method of claim 1, wherein X and Y are equal.
 7. The method ofclaim 1, wherein the pre-interference processing mitigates interferencein a presence of an additive white Gaussian noise (AWGN) vector.
 8. Themethod of claim 1, where the pre-interference processing is based on alinear operation multiplying the Y antennae uplink signals with a matrixor a linear combination of the Y antennae uplink signals.
 9. The methodof claim 1, wherein the ground antennae are interconnected via a fiberor microwave link.
 10. The method of claim 1, wherein thepre-interference processing is based on high-quality channel stateinformation (CSI) about the propagation of the MIMO transmission.
 11. Amethod for providing Multi-Input Multi-Output (MIMO) feeder downlinksfor a satellite, the method comprising: separating X satellite antennaeon the satellite; disposing Y ground antennae such that a MIMOtransmission by the X satellite antennae is spatially orthogonal upon aMIMO reception of Y antennae downlink signals by the Y ground antennae;and post-interference processing the Y antennae downlink signals togenerate Y gateway downlink signals that minimize channel interferencefor the MIMO reception at the Y ground antennae, wherein X is greaterthan 1, Y is greater than 1, the Y ground antennae are dominantline-of-sight (LOS) of the X satellite antennae, and a channel capacityof the MIMO transmission is greater than a channel capacity of XSingle-Input Single-Output (SISO) transmissions.
 12. The method of claim11, further comprising transmitting the MIMO transmission via the Xsatellite antennae; and receiving the MIMO transmission as the Yantennae downlink signals.
 13. The method of claim 11, wherein the Yground antennae are spaced within an acceptable range of an optimalposition.
 14. The method of claim 11, wherein the satellite comprises aGeosynchronous Earth Orbit (GEO) satellite having a bent-pipe design, anon-board processing design, a transparent payload design, a regenerativepayload design or a combination thereof.
 15. The method of claim 11,wherein X and Y are equal.
 16. The method of claim 11, wherein thepost-interference processing mitigates interference in a presence of anadditive white Gaussian noise (AWGN) vector.
 17. The method of claim 11,where the post-interference processing is based on a linear operationmultiplying the Y antennae downlink signals with a matrix or a linearcombination of the Y antennae downlink signals.
 18. The method of claim11, where the post-interference processing is based on one or more of, aweighted or non-weighted version of, a zero-forcing (ZF) criteria, aminimum mean-square error (MMSE) criteria, or a regularized ZF (RZF)criteria.
 19. The method of claim 11, wherein the ground antennae areinterconnected via a fiber or microwave link.
 20. The method of claim11, wherein the post-interference processing is based on high-qualitychannel state information (CSI) about the propagation of the MIMOtransmission.